A duplexer is a device that enables the simultaneous transmission and reception of signals via an antenna. A duplexer includes a transmit filter and a receive filter. The transmit filter passes energy in a transmit band from a transmitter to the antenna and blocks energy outside of the transmit band. The receive filter passes energy in a receive band from the antenna to a receiver and blocks energy outside of the receive band. A duplexer may include a quarter wave transmission line that provides a 90 degree phase shift that prevents the receive filter from excessively loading the transmit filter.
A duplexer is commonly implemented to match its impedance with the impedance of an antenna connected to the duplexer. For example, an antenna connected to a duplexer typically has an impedance of 50 Ohms. As a consequence, the transmit and receive filters in a duplexer are typically implemented with 50 Ohms impedances. In addition, a quarter wave transmission line in a duplexer is typically implemented with a 50 Ohm impedance.
A duplexer implemented with a relatively low impedance receive filter, e.g. 50 Ohms, may have a variety of disadvantages. For example, a receiver that connects to the receive filter of a duplexer may have a relatively high input impedance. As a consequence, a matching network may be needed between the receiver and the receive filter of the duplexer. Unfortunately, a matching network may increase the cost of a receiver and may cause a signal loss into a receiver. In addition, a low impedance receive filter in a duplexer may impose a lower limit on the overall size of the receive filter, thereby further increasing the cost.